Principle

An investigation of binocular vision can be incorporated into an investigation of the motor system and an investigation of the sensory system. During investigation of the motor system abnormal position (strabismus) and abnormal movement (disorders of co-ordinated movement, restrictions in the visual field) should be diagnosed and measured. During investigation of the sensory system the superimposing of the visual impressions of both eyes (fusion) and spatial vision (stereoscopic vision) are tested.

When conducting an investigation of ocular motor disturbances or a diagnosis of strabismus abnormal positions of the eyes are measured. Abnormal positions of this kind can be caused by hyper or hypofunction of one or more of the six eye muscles or by a disorder of one or more of the three nerves that control muscle functions. In order to assess disorders of the eye muscles strabometry is performed in various directions of gaze.

For the most accurate possible assessment of eye muscle function testing in as many directions of gaze as possible is necessary. In mathematical terms an infinite number of directions of gaze is possible. Testing of the nine main directions of gaze is sufficient for a basic diagnosis and makes sense in physiological terms. These are as follows: left and up, up, right and up, left, straight ahead, right, left and down, down, right and down. The deviations in the directions of gaze of both eyes can be expressed in angular degrees or in prism diopters.

Eye muscle functions can also be tested by what is known as co-ordimetry. This means that strabismus is illustrated by graphical representation in a diagram of deviations in directions of gaze in both eyes. Testing is performed with what are known as “tangent plates”. Their horizontally and vertically curved lines are produced by projections of the surface co-ordinates of a sphere onto a surface situated at the same distance as the radius of the sphere. The subject’s head is situated at the centre of the sphere, frontoparallel to the projected area. What are known as “tangent plates” are always calculated only for a certain distance. Fixation points for various directions of gaze are derived from the centre of the screen and from the points of intersection of certain horizontal and vertical lines. The image can be separated in widely differing ways. The most usual way is to separate the image by means of fixation objects in complementary colours and filter goggles in complementary colours.

With the Hess plate, separation of the image from the two eyes is performed, in the same way as with the original Hess-Screen, by filter goggles in complementary colours (HESS, W.R.: A new examination method in cases of double image. Arch. Augenheilk. 62 (1909) 233).

The Hess co-ordinates are calculated for an examination distance of 50 cm. They are derived mathematically in the following way: the eye of the observer is situated in the middle of a (imaginary) sphere and looks vertically at the screen which is placed at a distance of r = 0.5 meters (= radius of the sphere). The co-ordinates are hyperbolae, which result from the projection of concentric circles (the parallels of latitude of the sphere).